Testosterone does more than drive libido and mood, it is one of the body’s most powerful regulators of muscle and fat. For men noticing a slow shift toward more belly fat and less strength despite training, the relationship between testosterone and body composition is often at the center of the story. This article looks at what the science says about testosterone replacement therapy (TRT), lean mass, fat mass, and fitness, and how objective testing can help you and your clinician make sense of it.

This post is shared across Maze Medical Fitness Testing and Maze Men’s Health, because body composition sits exactly where performance testing and men’s hormonal health meet.

The Testosterone – Body Composition Connection

Body composition, the ratio of lean (muscle) mass to fat mass, is tightly linked to testosterone levels. Population data show that higher testosterone is associated with greater lean mass and lower fat mass, while low testosterone tracks with increased fat, particularly visceral (abdominal) fat (Liu et al., Clinical Endocrinology, 2025).

The relationship runs in both directions. Low testosterone promotes fat gain, and excess fat, especially visceral fat, further lowers testosterone, partly by converting testosterone to estrogen. This creates a self-reinforcing loop that can be hard to break with lifestyle change alone (Saad and Traish, American Journal of the Medical Sciences, 2025).

What TRT does to Muscle and Fat

In men with genuine testosterone deficiency, restoring testosterone to normal levels has consistent, measurable effects on body composition:

• Increases lean/muscle mass. Testosterone is a direct anabolic signal to skeletal muscle, increasing muscle protein synthesis and muscle fiber size (Bhasin et al., Endocrine Reviews, 2026).
• Reduces fat mass. Trials and reviews report reductions in total and fat mass with therapy, alongside gains in lean mass (Delbarba et al., Reviews in Endocrine and Metabolic Disorders, 2026).
• Improves strength and some measures of physical function, though the size of the functional benefit is more modest than the change in muscle mass (Bhasin et al., JAMA Internal Medicine, 2026).

An important nuance: the change in how you look and weigh can be misleading. Because muscle is denser than fat, scale weight may barely move even as fat falls and muscle rises. This is exactly why body composition testing matters more than the bathroom scale.

Testosterone is not a Substitute for Training

One of the most important messages for men considering TRT is that testosterone and exercise are not interchangeable. In middle to older aged men with low-normal testosterone, testosterone and exercise act on different aspects of health: exercise drives cardiorespiratory fitness (aerobic capacity), while testosterone’s main effect is on body composition and strength (Chasland et al., American Journal of Physiology, 2021; Green et al., Journal of Clinical Endocrinology & Metabolism, 2023).

In other words, TRT can help rebuild muscle and shed fat, but it does not replace the cardiovascular and metabolic benefits of structured aerobic and resistance training. The best results come from combining hormonal optimization (when clinically indicated) with a consistent exercise program.

Metabolic Ripple Effects

Because visceral fat drives insulin resistance, the body composition shifts seen with testosterone therapy have downstream metabolic implications. In men with hypogonadism and prediabetes, testosterone therapy was associated with a lower rate of progression to diabetes in a substudy of the large TRAVERSE trial (Bhasin et al., JAMA Internal Medicine, 2024). Reviews similarly link testosterone deficiency to components of the metabolic syndrome (Saad and Traish, American Journal of the Medical Sciences, 2025). Importantly, cardiovascular safety data continue to evolve, and testosterone should be viewed as one part of cardiometabolic risk management, not a stand-alone fix (Yeap and Anawalt, Journal of Clinical Endocrinology & Metabolism, 2025).

Why Testing Matters Before and During Therapy

TRT should only be started for a genuine, confirmed diagnosis. Both the Endocrine Society and the American Urological Association recommend diagnosing testosterone deficiency on the basis of consistent symptoms plus unequivocally low morning testosterone levels confirmed on two separate measurements — not symptoms alone (Bhasin et al., Journal of Clinical Endocrinology & Metabolism, 2018; American Urological Association Guideline, Journal of Urology, 2024).

Objective testing is just as valuable for tracking whether therapy and training are actually working:

• Body composition testing (e.g., DXA) quantifies lean mass and fat mass and regional fat distribution far more accurately than weight or BMI, and is the tool of choice for following change over time (Delbarba et al., Reviews in Endocrine and Metabolic Disorders, 2026).
• Fitness and performance testing captures the aerobic and strength gains that come from training — the piece TRT alone will not deliver.
• Bloodwork confirms the diagnosis, guides dosing, and monitors safety.

At Maze Medical Fitness Testing, performance and body-composition testing give you a clear baseline and a way to measure real progress. At Maze Men’s Health, that data is paired with a thorough hormonal evaluation so that any decision about TRT is grounded in evidence rather than guesswork.

The Takeaway for Men

• Testosterone and body composition are tightly and bidirectionally linked — low testosterone and excess fat feed each other.
• In men with confirmed deficiency, TRT reliably increases muscle and reduces fat, with more modest gains in strength and function.
• TRT is not a replacement for exercise, aerobic and resistance training deliver benefits testosterone cannot.
• Objective testing (body composition, fitness, and labs) is the key to diagnosing correctly and proving that therapy and training are working.

If you are noticing changes in strength, body composition, energy, or performance, combining a men’s health evaluation with objective fitness and body composition testing is the most reliable way to understand what is driving it.

Learn more and book your consultation at Maze Men’s Health Today!